During the transfer of unit loads from one conveyor onto a second conveyor arranged at an angle with respect thereto, it is frequently necessary to ensure that the orientation of the unit load on both conveyors corresponds to certain criteria, for example that the unit load is not displaced or rotated on the second conveyor. This is particularly important if the second conveyor is a supply and/or acceleration band upstream of a cross-belt or tilting-tray sorter. As is known, cross-belt or tilting-tray sorters comprise individual, interconnected conveyor elements which are connected to one another to form a train or an endless belt and on which a respective unit load is transported. For the positionally correct transfer of the unit load parts onto the sorter elements, it has to be ensured that the unit load is transferred in a manner matched to the supporting surface of the sorter elements. This can be brought about in the simplest manner if the unit load coming from the transfer band of the feed conveyor already rests on the supply band in a defined manner.
Conveyors of the type described at the beginning are customarily arranged in such a manner that their transport directions run at an obtuse angle with respect to one another. In addition, the conveying speeds of the conveyors involved are coordinated with one another; the unit loads retain their predetermined orientation on the supply band during the transfer, and the transfer angle between supply band and cross-belt or tilting-tray sorter is selected accordingly. Generally, an acute angle is enclosed between the supply band and the cross-belt or tilting-tray sorter.
Known feed conveyors of the type in question have a transfer band which is kept very flat and projects at least in a subregion over the conveyor belt of the second conveyor. Both conveyors are operated at the same speed or at a speed which increases in the conveying direction. However, it has been shown that, as a consequence of the transporting movement of the conveyors, air turbulence which has the effect of an air cushion is produced directly above the surface of the second conveyor. This leads to letters having a low weight and large area, for example DIN A4 envelopes, when “shooting” from the transfer band onto the conveyor belt of the second conveyor, being carried away by the air cushion and landing at undefined locations, which understandably has negative consequences with regard to the further transportation. Since the unit load which comes to lie on the belt band in an undefined manner cannot be transferred to the sorter in the required orientation, disturbances are unavoidable.
The problem is intensified by the fact that the transfer band and the belt conveyor are arranged at an angle with respect to each other. On account of the abrupt change in direction during shooting of the unit load onto the conveyor belt of the second conveyor, the unit load is offset and/or rotated by means of the pulse, which is associated with the change in direction, on the air cushion described. Even in the case of relatively large unit load parts, this leads to an imprecise transfer onto the cross-belt or tilting-tray sorter, and therefore the operational capacity and the availability of the system are impaired.
DE 103 46 122 A1 describes a transfer apparatus for a sorter of the type in question, in which the unit load is deposited at the end of the transfer band of a feed conveyor onto a movable table which adjoins it in the conveying direction and which is displaced together with the unit load resting on it by the belt of the supply band. So that the unit load does not slip from the table during the movement of the table and the acceleration and braking phases required therefor, it is held on the table by means of a vacuum generated therein and is not ejected until at the discharge point by the negative pressure being cancelled and by abrupt movement back of the table. However, this solution does not prevent the problem outlined at the beginning of an air cushion forming on the conveyor belt, although the shooting in of the unit load and the change in direction which arises at the same time are less significant in this solution. Nevertheless, the known solution is highly complicated and, because of the many mechanical parts, is susceptible to faults and is expensive.